The production of metals, such as transition and rare earth metals, has always presented several technical challenges. With specific reference to a particularly useful transition metal, titanium is the ninth most abundant element and possesses unique and desirable properties, such as high melting point, high corrosion resistance and the ability to form lightweight alloys, but it has not been used widely owing to its production costs. Titanium dioxide, which finds widespread use as a white pigment in paints, is readily available in the Earth's crust, but the separation of titanium metal from the oxygen in titanium dioxide has traditionally presented several challenges, in terms of time and energy requirements and handling difficulties associated with corrosive and volatile reagents and by-products.
Typically, the extraction of highly reactive metals requires the use of expensive electrolysis methods1-15. The most commonly used processes for the production of titanium, however, are reductive processes. The Kroll process uses ilmenite or rutile as a starting material and this is carbo-chlorinated to obtain titanium tetrachloride, which is then reduced using magnesium metal. The magnesium chloride which is thus obtained is separated by distillation. This process, however, is time-consuming and takes several days for completion. Hunter's process is similar to the Kroll process, but uses sodium, rather than magnesium, to effect the reduction of titanium tetrachloride. The FFC process, which was developed at the University of Cambridge, is also extremely time-consuming and involves the reduction of titanium dioxide pellets in a molten calcium chloride bath. However, despite extensive development work over a period of years, this process still fails to achieve complete removal of the oxide layer16,17.
Alternative lengthy on-going research efforts have also failed to arrive at a cheaper production route. Several researchers, for example, have attempted electro-deposition of titanium from ionic solutions but have faced difficulties in eliminating multivalent titanium ions and highly reactive dendrite products4-8.
Reductive processes for the manufacture of titanium metal from titanium dioxide typically encounter difficulties associated with the presence of various lower oxides or Magnéli phases in the TiO2, since titanium can exist in several oxidation states that make the reduction more complicated and difficult. The present inventors have, however, successfully addressed this issue and have effectively reduced all the lower oxidation states of titanium, thereby allowing for the production of very high purity titanium metal.
Specifically, the inventors have examined the direct de-oxidation of titanium dioxide using calcium metal in order to produce titanium metal and have provided a process which is simple and rapid when compared with conventional methods and facilitates the production of titanium metal which is free from oxygen impurity whilst allowing for massive reductions in production costs. The approach which has been developed has been found to be applicable to the production of a wide range of other metals, most particularly other transition and rare earth metals.